The goal of this project is basically two-fold. First, laboratory synthesis of Costuslactone and of its isomers would confirm the structure and establish the complete stereochemistry of these compounds. Like other guaianolides of similar structure, Costuslactone or one of its isomers may show useful biological properties such as cytotoxic or anti-tumor activity when broadly screened. Furthermore, several of the intermediates in this synthesis, of even Costuslactone itself, may be transformed into other guaianolides (e.g., diepoxy or alpha-methylene-gamma-butyrolactone guaianolides) potentially having higher activity in living cells. The second aspect of especial significance is the accumulation of experimental data concerning the general course and stereochemistry of reactions in conformationally flexible hydroazulene systems. The relatively low energy barriers separating isomers of any particular hydroazulene weaken predictions of which conformer will be the most reactive and which configuration will be the most thermodynamically stable. Empirical data on the stereochemistry of reactions in the Costuslactone system may lead to a more detailed understanding of these reactions and may lead eventually to stereoselective syntheses of a variety of guaianolides and pseudoguaianolides.